1. Field of the Invention
The present invention relates to an optical sheet for changing the travel direction of light, to an optical member and a surface light source device which use the optical sheet, and to a liquid crystal display device which uses the surface light source device. In particular, the present invention relates to an optical sheet having an excellent scratch resistance, and to an optical member, a surface light source device and a liquid crystal display device which use the optical sheet.
2. Description of the Related Art
An optical sheet is known which is disposed on the light exit surface of a backlight source in a transmission type liquid crystal display device and which condenses exiting light and increases the luminance.
For example, JP 3518554B discloses an optical sheet whose back surface, opposite from a prism surface formed by an arrangement of triangular unit prisms as unit optical elements, is roughened by the provision of a large number of void-forming micro-protrusions having a height of not less than the wavelength of source light and not more than 100 μm. Thus, the back surface of the optical sheet, opposite from the prism surface, is not a mere smooth surface but such an uneven surface (rough surface). When a light guide plate is disposed adjacent to the back surface of the optical sheet, the uneven surface can prevent optical adhesion (optical contact) to the light guide plate, thereby effectively preventing non-uniform in-surface distribution of luminance, the formation of an interference pattern, etc., which would be caused by the optical adhesion.
Such an uneven surface with a large number of micro-protrusions can be formed e.g. by a hot embossing method, a molding method (2P method: photopolymer method) using an ultraviolet or electron beam curable resin and a mold, or a coating method which involves applying a coating composition, including a resin solution containing fine particles, onto a surface to produce a coating having unevenness formed by the fine particles. Of these, the coating method has the advantages that a thermoplastic or thermosetting resin can be used as the resin and resin beads or the like can be used as the fine particles, and an uneven surface can be formed at a low cost in a simple manner as compared to other methods.
While an optical sheet having such a rough back surface can prevent optical adhesion, the optical sheet has a problem in that micro-protrusions on the uneven surface, fine particles that have escaped or fallen off from a coating, etc. can scratch the surface of another optical member disposed adjacent to the rough back surface.
In order to prevent scratching on an adjacent optical member, JP 3913870B proposes a technique of using monodispersed spherical beads, whose half-value width in the particle size distribution is not more than 1 μm, as fine particles contained in a coating.
It is also important for an optical sheet not to lower the luminance by its presence. JP 8-286005A proposes an optical sheet having a lens surface on one side and which is provided with a low-refractive index layer on the other surface as a light entrance surface in order to prevent unnecessary reflection of light at the light entrance surface, thereby increasing the front direction luminance.
Even though scratching of another optical member may be improved by the use of monodispersed fine particles in the optical sheet as proposed by JP 3913870B, the problem of scratching on the optical sheet itself has yet to be solved.
The phenomenon where an optical sheet scratches not another optical member but the optical sheet itself firstly occurs when shipping the optical sheet as a product prior to assembling the optical sheet into a surface light source device. Optical sheets are usually produced in the form of a strip-shaped sheet from the viewpoint of productivity, and the strip-shaped sheet is rolled up into a roll. After storing and/or transporting the roll, the roll is cut into sheet pieces of a shape and size according to the intended use upon shipping. Optical sheets, in the form of sheet pieces, are stacked together during storage or transportation. When the optical sheets are in a rolled or stacked state as described above, the front surface (outermost surface) of one optical sheet and the back surface (facing the front surface) of another optical sheet superimposed on the one optical sheet are in contact with each other. The front and back surfaces, in contact with each other, rub against each other e.g. due to vibration during storage or transportation, which causes scratches on the surfaces, or forces fine particles out of one surface, which in turn causes scratches. Such scratching may occur on either one or both of a prism surface and a coating surface opposite from the prism surface (front and back surfaces).
Such scratching on the front and back surfaces which occurs before use of the optical sheets may be avoided by attaching a protective film to the front and back surfaces, and peeling the protective film off the surfaces when assembling each optical sheet into a surface light source device. From the viewpoint of low cost and resource saving, however, it is preferred not to use such a protective film which finally becomes unnecessary.
The phenomenon where an optical sheet scratches not another optical member but the optical sheet itself secondly occurs after assembling the optical sheet e.g. into a surface light source device. For example, the phenomenon can occur when assembling two stacked optical sheets as described e.g. in Japanese Patent Publication No. H1-37801 and Published Japanese Translation No. H10-506500 of International Patent Publication No. WO 96/10148. In general, the two stacked optical sheets each have, in one surface, triangular prisms as unit optical elements arranged in an arrangement direction, and the two optical sheets, facing in the same direction, are stacked together such that the arrangement direction of the unit prisms of one optical sheet is perpendicular to that of the other optical sheet.
In a surface light source device having a stack of optical sheets disposed adjacent to each other, or in an optical device, such as a liquid crystal display device, which uses the surface light source device, scratching on the front and back surfaces of the optical sheets can occur due to vibration even after assembly. This is because an optical device is sometimes subjected to vibration when it is stored or transported as a semifinished product or a commercial product.
Also when an optical sheet is disposed adjacent to and in contact with another optical member other than an optical sheet, such as a light guide plate or a liquid crystal panel, in an optical device, the front or back surface of the optical sheet itself can be scratched due to vibration during storage or transportation of the optical device.
Unit optical elements such as prisms can receive an external force in a relatively wide area and, in addition, do not contain matter that can escape or fall off, such as fine particles. Therefore, it is possible to design unit optical elements so that they can avoid scratching due to an external force by using a flexible and restorable resin as a material for the unit optical elements, as described in JP 2009-37204A. With reference to an uneven coating surface, on the other hand, stress concentrates in a relatively narrow area. In addition, the coating contains fine particles which can escape or fall off. It has therefore been difficult to prevent scratching on such a coating surface even when restoring properties are imparted to the coating.
Besides roughening of a surface of an optical sheet in order to prevent the problem of optical adhesion, it is also possible to deal with the problem by roughening the surface of another optical member which is to make contact with the optical sheet, as described in the above-cited JP 3518554B, paragraph [0015] and FIG. 4. For example, when an optical member which is to make contact with an optical sheet is a light diffusing sheet, the light exit surface (and the light entrance surface) of the light diffusing sheet is roughened.
In this case, it is not necessary to roughen the back surface (coating surface) of the optical sheet, and the smooth back surface can reduce scratching on the adjacent prism surface (front surface) of the light diffusing sheet or the adjacent surface of another optical member. Even if the adjacent prism surface or the adjacent surface of another optical member is scratched, the degree of scratching will be relatively low and will not significantly affect the optical properties.
However, a scratch is fairly noticeable on the back surface of the optical sheet because of the smoothness of the surface. It is highly likely that even a small scratch which does not affect the optical properties will be determined as a defect by visual inspection, or will be deemed to lower the commercial value of the optical sheet. On the other hand, a scratch is hardly noticeable visually on the prism surface of the optical sheet because of the ribbed appearance and the light condensing or diffusing properties of the prism surface. Therefore, such a small scratch as not to affect the optical properties could be allowable.
Thus, even when a back surface, opposite from a prism surface, is a smooth surface, the problem of scratching due to friction between the front and back surfaces of optical sheets remains. Reduction of scratching on the back surface of an optical sheet is of particular importance and must be addressed when no use of the above-described protective film is intended.
A low-refractive index layer, which is provided in an optical sheet from the viewpoint of increasing the luminance, in most cases has a small thickness such as about ¼ of the wavelength of light whose reflection is to be prevented, and is susceptible to scratching due to contact with another optical member. Furthermore, such a low-refractive index layer does not take account of prevention of optical adhesion and prevention of scratching on another optical member and the optical sheet itself. Therefore, scratching on the optical sheet and an adjacent optical member or sheet cannot be prevented.
The provision of a coating on the back side of an optical sheet necessitates a process and a material for the formation of the coating. Thus, from the viewpoint of reduction in the product cost, it is highly desirable if the problem of scratching on an optical sheet can be dealt without provision of a coating.
It is therefore an object of the present invention to provide an optical sheet including an optical element surface, formed by e.g. prisms, and a back surface opposite from the optical element surface, and having excellent scratch resistance.
It is also an object of the present invention to provide a surface light source device and a liquid crystal display device which use the optical sheet and in which optical members, including the optical sheet, are hardly scratched owing to the use of the optical sheet.